Dating Prograde Amphibolite and Granulite Facies Metamorphism Using In Situ Monazite U‐Pb SHRIMP Analysis

Abstract

In situ SHRIMP U‐Pb analysis of monazite grains from pelites within an early‐formed high‐temperature shear zone in the southern Broken Hill Block, Australia, has been used to constrain the timing of prograde amphibolite facies and peak granulite facies metamorphism within the terrain. Geochronological analyses from grains inhabiting texturally distinct localities revealed two distinct age populations, ca. 1619 and ∼1600 Ma. The older age was obtained from grains that occur as inclusions completely enclosed within coarse‐grained K‐feldspar and garnet grains, which evidently armored the monazite inclusions against resetting during younger deformation and metamorphic events. The ca. 1619‐Ma monazite population occurs as part of an amphibolite facies inclusion assemblage hosted within the peak granulite facies mineral assemblage and constrains the timing of prograde amphibolite facies metamorphism within the Broken Hill Block. The younger ∼1600‐Ma monazite age population was from grains within the pervasive shear fabric of the pelites or adjacent to fractures or grain boundaries in the matrix. The ∼1600‐Ma age population represents either a later stage of monazite growth or isotopic resetting during deformation along the high‐temperature shear zone at peak granulite facies conditions.